1 definition
by
Sherman Tank

Virii defines a category of software that is comprised of viruses, worms and trojans. The general usage of the word is to describe a collection of software.
Virii itself is a plural word as there is no need for a singluar form because the more descriptive singular form of {virus, worm, or trojan} could be used.
Virii is not the plural form of virus (which is viruses). Virii has nothing to do with biological viruses.

Correct: I have a huge collection of virii!
Incorrect: I got a virii on my computer.